Developer



3,072,541 Patented Jan. 8, 1963 DEVELOPER Benjamin L. Shely, Mahtomedi,and Bryce L. Clark, St.

Paul, Minn., assignors to Minnesota Mining and Manufacturing Company,St. Paul, Minn., a corporation of Delaware No Drawing. Filed Oct. 17,1958, Ser. No. 767,788

5 Claims. (Cl. 204-18) This invention relates to the formation ofpermanent visible reproductions of light-images on photoconductivesurfaces by methods involving electrolysis at the exposedlight-sensitive surface. It has particular reference to improvedelectrolytic developer solutions employed in such method.

- Methods have recently been devised for electrolytically formingvisible reproductions of light-images on strongly photoconductivelight-sensitive copy-sheet materials. Apparatus for carrying out suchmethods, particularly in making enlarged reproductions if microfilmcopies of graphic originals, has also been made available. Typically,there is involved the brief irradiation of a sensitive copy-paper with alight-image corresponding to the copy desired, and the subsequentelectrodeposition at the con ductive light-struck areas of a metallicimage-forming material as the copypaper is withdrawn from theirradiation area. The copy is obtained in substantially dry form, sinceonly a surface application of developer solution is applied. Nosubsequent treatment, such as heating, washing, or subjecting to theaction of chemical vapors, is required. Completed reproductions ofmicrofilm transparencies or other graphic originals may be produced at arate of ten or more per minute.

A recommended form of light-sensitive copy-paper for use with theapparatus and method hereinabove identi fied consists of a stronglyphotoconductive zinc oxide surface coating on a metallized or metallicconductive backing or sheet material. Such a sheet, having an 0.8 milcoating of four parts by weight of zinc oxide and one part of polymericinsulating binder on a laminate of paper and aluminum foil, typicallyhas a conductivity value, measured as hereinafter described, of at leastabout mho/cm. and preferably not less than about 10* mho/crn. Underequilibrium dark conditions the conductivity value is not higher thanabout one-twentieth of the conductivity value under illumination, andordinarily is much lower.

The conductivity value is measured as follows: A small sample of thesheet material is insulated at back and edge areas with a nonconductivewaterproof covering, e.g., of plastic adhesive tape, an electricalconnection to the conductive metallic substrate being provided. Thesample is suspended in a transparent glass cell containing 200 ml. of asolution of tenth-molar ammonium sulfate and facing an open frameelectrode serving as the anode. Current flow per unit area through themeasured thickness of the coating under an applied voltage is measuredboth with the sample under equilibrium dark conditions and whenilluminated. A potential of 10 volts is convenient but not critical.Values at several thicknesses of coatings may be determined and thevalue at a standard thickness of 0.8 mil obtained by interpolation.Illumination is provided from a 500 watt incandescent-filament lamp,i.e. at an intensity of about 1300 foot-candles. The photoconductivityvalue is calculated from the values thus obtained.

As electrolytic developer solutions for use in the aboveidentifiedapparatus and method, there have been suggested solutions of salts ofplatable metals, e.g. copper sulfate, nickel chloride, or cadmiumnitrate solutions. An initially visible image may indeed be formed witheach of these; but for one reason or another the resulting copy has notbeen entirely satisfactory. For example, with platable base metals theimage is found to fade rapidly when the copy is exposed to highhumidity. Since some moisture is unavoidably present at the surface ofthe sheet following image development, conditions are normallyunfavorable for image permanency, Furthermore, although electrolysis ofmany such salts does initially produce a visible image, the quality ofthe image is low. The image areas obtained are in most cases grayishrather than the desired dense black, and the copy lacks contrast. Thenoble metals produced no better, or even less effective images. Silvernitrate, for example, produces a yellowish-brown image and a darkenedbackground. Gold salts likewise produce images of low density. Solutionsof gold and platinum salts are highly corrosive to metallic structuralcomponents and prohibitively expensive for most copying purposes.

Electrodeposition of metals from corresponding salt solutions, as in theelectroplating art, is frequently improved in one respect or another bythe inclusion of variout additives in the plating solution. For example,soluble cyanides are normally added in small amounts to silver-platingbaths to improve the brightness and permanence of the plate. It mighttherefore be expected that the substitution of typical metal-platingsolutions for the simple salt solutions would likewise improved thequality of the image obtainable in the electrolytic reproduction processhereinabove identified. It has been found, however, that electroplatingformulations in general offer inadequate improvement over the lesscomplicated simple salt solution in such process. Formulations employingbase metals provide image areas of somewhat improved initial density andcontrast, but the images are not permanent and fade or disappear rapidlywhen the sheet is held at high humidities. The cyanides are additionallyhighly undesirable because of their poisonous nature, since the residualunreacted salts, or residues resulting from the electrolyticdecomposition of such salts, in all instances remain on the copy-paperrather than being removed, as in normal metal plating operations, bysubsequent washmg.

On the other hand, additives which are employed to provide increasedbrightness of the metallic plate might appear likely to bedisadvantageous in the electrolytic development of light-images on awhite base, where a dense black rather than a bright metallic deposit isnormally desired.

It has now been found that certain normally stable solubleelectrolyzable complexes of platable metals provide electrolyticdeveloper solutions with which by methods noted hereinbefore, may beobtained dense darkcolored image-forming deposits of a high degree ofperrnanence, all as demonstrated by the following illustrative butnon-limitative specific examples.

The several components are mixed together at room temperature, forming aclear solution, which is placed in the metal reservoir of anelectrolytic printing device. A thin strip of cellulosic sponge ispartly submerged in the solution. A sheet of photoconductive zinc oxidecoated copypaper as described hereinbefore is exposed to a lightimage,and the exposed surface then drawn slowly across the strip of spongewhile the conductive backing of .the sheet and the metal reservoir areconnected to the poles of a battery. The sheet is thereby made thecathode of an electrolytic cell. As the sensitized sheet contacts thesolution in the sponge, a dense black deposit is formed at theconductive light-struck areas. The print shows no visible decrease incontrast or image density after several weeks in a high humidity testchamber at normal room temperature.

Pressure between sheet and sponge, width of sponge face, and otherfactors are so controlled as to provide a minimum deposit of solution onthe sheet consistent with the eifective formation of the image. Theamount applied is insufficient to appear as a liquid layer on the sheetas it is taken from the printer. The moisture is soon volatilized, theresidual non-volatile material remaining on or in the oxide-coatedsurface.

When a dilute solution of silver nitrate is employed in the foregoingprocedure as the electrolytic developer solution, the image areas areyellowish-brown in color and the background areas are noticeablydarkened. The resulting reproduction is essentially stable, showing noobseivable change or prolonged storage at high humidity, but has lowcontrast and is not a desirable print.

When combined as described in Example 1, silver nitrate and thioureaform a normally stable, soluble, electrolyzable complex which can beseparated from the solution in crystalline form by cooling to somewhatbelow C. The washed and dried crystals are found to melt at about 156 C.and to be substantially free of acetamide. Dissolved in appropriateionizable solvent, the pure crystalline complex likewise provides astable liquid electrolytic developer solution useful in formingpermanent dense dark-colored image-forming deposits on photoconductivecopy-paper.

In the absence of thiourea, the acetamide shown in Example 1 is itselfinstrumental in increasing the density and improving the appearance ofthe image-forming deposits obtained with the silver nitrate developersolution. However the complexes formed with this compound are relativelyless stable and presumably are not present in the solution of theexample, although such complexes may be present when less than threemols of thiourea are employed. The acetamide is in any event helpful informing the stable complex from silver nitrate and thiourea and, sinceit imparts no harmful efiiects, is ordinarily retained in the developersolution.

Maximum image density is obtained with the thioureasilver nitrate systemat the three-to-one molar ratio. Increasing the amount of thiourea, forexample from 1.35 up to 4.0 parts in Example 1, does not impair eitherthe density or the permanence of the image-forming deposits whenproperly formed, but has caused noticeable fading in cases where largerthan usual amounts of the solution were applied and allowed to remain onthe copy-paper.

The image-forming deposits obtained with the thioureasilver nitratecomplex have been observed to be composed primarily of metallic silver(or its oxide) and silver sulfide. The image areas therefore have muchin common with images produced by silver halide photography; and theappearance of the finished prints furthers this comparison. For thisreason, stable soluble electrolyzable complexes of silver salts andsulfur-containing complexing agents are ordinarily preferred. Complexesof other platable metals with these and other complexing agents, andwhich are stable in solution form and on electrolysis produce permanentdark-colored dense image-forming deposits, are also useful. Aqueoussystems will ordinarily be found preferable, but other ionizing solventsare known with which effective solutions of many of these variouscomplexes may be prepared. For example, glycerine and formamide may beused where the amount applied is so small in relation to the thicknessand absorbency of the copy-paper as to avoid any necessity of subsequentsolvent evaporation.

Example 2 A 2% solution of (HAuC1 -3H O) is applied in a thin coating ona sheet of photoconductive zinc oxide coated copy-paper which has beenexposed to a lightimage. The solution is applied by wiping with acellulosic sponge dampened with the solution and in conductive contactwith a gold electrode through which contact is made to a suitable sourceof potential and to the conductive backing of the copy-paper. Platingoccurs at the cathodic light-exposed areas, which are thereby renderedvisible. When tested with a Densichron optical density measuringinstrument, the proportion of the incident light normally reflected bythe untreated copy-paper which is absorbed by the thus developed imageareas is found to be 37%. Although the image is stable, remainingunchanged in density after one week at high humidity, the density isundesirably low for effective readability.

An aqueous olution prepared with 2% of gold chloride in conjunction with4% of acetamide is similarly tested. The image areas have a density of68% as first prepared, and 67% after one week at high (98%) humidity.Addition of 2% thiourea to the solution results in the formation ofdeveloped image areas having a density of 81% and likewise dropping onlyto after one week at high humidity. In both instances excellentreproductions are obtained, and the image areas remain dark-colored anddistinct in the presence of substantial residues of develo-per materialsand at high humidities.

What is claimed is as follows:

1. The process for producing permanent, dense imageforming deposits onlight-exposed areas of strongly photoconductive photosensitivecopy-paper having a photoconductive coating on an electricallyconductive backing, said copy-paper being capable of electrolytic imagereproduction thereon, which comprises electrolyzing at the conductivelight exposed areas of said photoconductive coating a stable liquidelectrolytic developer solution consisting essentially of a stablesolution in an ionizing solvent of a soluble electrolyzable complex of asilver salt and a thiourea in a 1:3 molar ratio, said electricallyconductive backing being connected as cathode.

2. The process of claim 1 in which said ionizing solvent is water.

3. The process for producing permanent, dense imageforrning deposits onlight-exposed areas of strongly photoconductive photosensitivecopy-paper having a photoconductive zinc oxide coating on anelectrically conductive backing, said copy-paper being capable ofelectrolytic image reproduction thereon, which comprises electrolyzingat the conductive light exposed areas of said photoconductive coating astable liquid electrolytic developer solution consisting essentially ofa stable solution in an ionizing solvent of a soluble electrolyzablecomplex of a silver salt and a thiourea in a 1:3 molar ratio, saidelectrically conductive backing being connected as cathode.

4. The process of claim 3 in which said ionizing solvent is water.

5. The process of claim 3 in which said electrolytic developer solutionalso contains acetamide.

References Cited in the file of this patent UNITED STATES PATENTS1,857,507 Hickman et a1 May 10, 1932 1,902,213 Brockway Mar. 21, 19331,903,860 Gockel Apr. 8, 1933 2,083,249 Thomson June 8, 1937 2,186,859Digby Jan. 9, 1940 2,443,119 Rubin June 8, 1948 2,692,190 Pritikin Oct.19, 1954 2,854,386 Lyman et al Sept. 30, 1958 FOREIGN PATENTS 151,971Germany Dec. 8, 1902 215,754 Australia June 23, 1958 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,072,541 January 8,1963 Benjamin L. Shely et al,

It is hereby certified that error appears in the above numbered patentrequiring correction and that the 'said Letters Patent should read ascorrected below.

Column 1, line 20, for "if" read of column 2, lines 20 and 21, for"variout" read various line 26, for "improved" read improve column 2,Example 1, line 4 thereof, for "Wate" read Water column 3, line 19, for"or" read 0n Signed and Sealed this 2nd day of July 1968,

(SEAL) Attesa' ERNEST w. SWIDER DAVID LADD Commissioner of PatentsAttesting Officer

1. THE PROCESS FOR PRODUCING PERMANENT, DENSE IMAGEFORMING DEPOSITS ONLIGHT-EXPOSED AREAS OF STRONGLY PHOTOCONDUCTIVE PHOTOSENSITIVECOPY-PAPER HAVING A PHA PHOTOCONDUCTIVE COATING ON AN ELECTRICALLYCONDUCTIVE BACKING, SAID COPY-PAPER BEING CAPABLE OF ELECTROLYTIC IMAGEREPRODUCTION THEREON, WHICH COMPRISES ELECTROLYZING AT THE CONDUCTIVELIGHT EXPOSED AREAS OF SAID PHOTOCONDUCTIVE COATING A STABLE LIQUIDELECTROLYTIC DEVELOPER SOLUTION CONSISTING ESSENTIALLY OF A STABLESOLUTION IN AN IONIZING SOLVENT OF A SOLUBLE ELECTROLYZABLE COMPLEX OF ASILVER SALT AND A THIOUREA IN A 1:3 MOLAR RATIO, SAID ELECTRICALLYCONDUCTIVE BACKING BEING CONNECTED AS CATHODE.